1. Field of the Invention
The present invention relates generally to circuits for processing a reproduced video signal. More particularly, this invention relates to a method for detecting a drop-out of a frequency modulated video signal and a drop-out detecting circuit for use in the reproducing system of a video tape recorder (VTR).
2. Description of the Prior Art
In the art it is known that in the reproducing system of a video tape recorder (VTR), there is employed a drop-out compensating apparatus for detecting whether or not a reproduced video signal contains a drop-out. According to this drop-out compensating apparatus, when the reproduced video signal contains a drop-out, the drop-out portion thereof is replaced with a signal of one horizontal period before, it is replaced with a signal of grey level or the level of an immediately-preceding signal is held in order to compensate for the drop-out.
As a drop-out detecting circuit used in the above-mentioned drop-out compensating apparatus, Japanese published patent gazette No. 57-27637 discloses such a drop-out detecting circuit that is arranged to detect a drop-out by detecting the frequency of a reproduced frequency modulated video signal.
When dust or the like enters the space between a magnetic reproducing head and a magnetic tape, so-called spacing loss occurs. This spacing loss lowers the level of a high band component of a reproduced frequency modulated video signal, causing a drop-out. At this time, in the reproduced frequency modulated video signal, the level of its lower side band goes high as compared with that of its carrier signal and the frequency component of the lower end of the lower side band becomes dominant so that this frequency is regarded as the carrier.
By effectively utilizing the above-mentioned features, the frequency component of the reproduced frequency modulated video signal which is regarded as the carrier is detected to detect the drop-out.
Meanwhile, when the video signal is emphasized and then frequency modulated in the recording system of the video tape recorder, there exists a peak portion of a video signal which is extended to the region lower than the black level with the result that the frequency of the carrier of the reproduced frequency modulated video signal is lowered to the frequency band of the side band lower than the frequency of the carrier corresponding to a video signal ranging from a sync. tip level to the white peak level.
In view of the above-mentioned aspect, it is proposed that a drop-out is detected by the fact that a frequency lower than that of a carrier corresponding to the peak portion of the video signal extended to the black level side and which is somewhat lower than the center frequency of the lower side band of the carrier corresponding to the video signal from the sync. tip level to the white peak level is taken as a reference frequency and that it is detected when the frequency of the reproduced frequency modulated video signal which is regarded as the carrier becomes lower than the reference frequency.
If the reference frequency is determined as set forth above, when upon playback with variable tape speed a magnetic reproducing head scans over two slant tracks across a guardband portion simultaneously, at the portion where the phases of the carrier signals reproduced from the two slant tracks differ from each other by 180.degree. C., these carrier signals cancel each other out. Thus, the levels thereof are considerably lowered, being regarded as a drop-out. At that time, the phases of the lower side band reproduced from two slant tracks are scarcely different by 180.degree. , and hence, the signal components thereof rarely cancel each other out, because the frequencies thereof are different from the carrier frequency. Thus, the level of the lower side band becomes higher than that of the carrier at that time, the level of the higher end of the lower side band is relatively high so that when the higher end of the lower side band is regarded as the carrier the frequency thereof becomes higher than the reference frequency. As a result, although the drop-out occurs, it cannot be detected.